Why do we Age?

Early theories on this subject (Medawar, 1952, Haldane, 1941) tried to understand how something as negative as ageing could have been positively selected for in evolution, especially since most animals in the wild do not live to old age. Only captive animals that are protected from predation, starvation or physical trauma, live to and die from old age. To give an explanation for this problem, Medawar hypothesised that there is a declining force of natural selection with age. Harmful genetic events that are expressed prior to sexual maturity of an individual will be strongly selected against, whereas the expression of such changes at later stages will not be subject to such negative selection. Later, the term antagonistic pleiotropy was contributed to the topic of evolutionary ageing (Williams, 1957). Antagonistic pleiotropy refines the ideas set by Medawar by suggesting that late-acting deleterious genes may be favoured by selection and be actively accumulated in individuals if they have beneficial effects early in life. Such beneficial effects may include the enhancement of an individual’s ability to survive until the reproductive period and/or to carry out reproductive activities in a successful manner.

Another explanation for the conflict between reproduction and longevity came much later with the Disposable Soma theory (Kirkwood, 1977). Unlike the antagonistic pleiotropy theory, the Disposable Soma theory does not explicitly implicate genes as being involved in the ageing process. It instead focuses on the distribution of precious metabolic resources for either increasing reproductive capacity or somatic cell maintenance (DNA repair, protein turnover and antioxidant defences etc). Therefore, organisms have evolved in such a way that the amount of energy invested in maintaining the somatic tissue is sufficient to keep the individual alive long enough to reproduce, but less than what is required to keep it alive indefinitely.